The established switching principle for switching and quenching higher currents in switchgears usually consists of a dual-breaking contact assembly which conducts the switching arcs arising therein over arc running rails in a stack of arc splitter plates in the form of deion chambers. In these chambers, the arcs are cooled and split into a plurality of sub-arcs, and this is linked to corresponding multiplication of the arc voltage. When the driving voltage is reached, the arc is quenched and the circuit is interrupted thereby. When switching high alternating currents, the arc quenching is usually assisted by dynamic magnetic blow-out fields, which are formed within the switchgear by suitably shaping the conductors. In order to quench direct currents, magnetic blow-out fields are usually used, however, which are generally produced by an arrangement of permanent magnets.
Unlike with the established alternating-current switchgears that have long been on the market, comparably large switchgears for interrupting low-frequency currents e.g. at 16⅔ Hz and direct currents accordingly have a greater load owing to the lower or lacking periodicity of the zero crossing of the current. The longer arc time arising therefrom ensures a higher energy content of the switching arcs in comparison with alternating-current switchgears. This leads both to contact material combusting more intensely and to a correspondingly high thermal load within the switching chamber. A thermal load of this type may reduce the insulation capacity within a switching chamber. As a result, this may reduce the electrical service life of the switchgear.
One option for reducing the load on a switchgear resulting from switching arcs is provided by what are known as hybrid switches. Known hybrid switches, such as those described in DE 103 15 982 A1, consist of a parallel circuit of an electromechanically actuatable main-switch contact assembly comprising a semiconductor switch e.g. based on a heavy-duty insulated gate bipolar transistor (IGBT). When switched on, said IGBT is high-resistance, and therefore the load current only flows via the closed mechanical contacts. During the switching-off process, the semiconductor switch is actuated such that it is low resistance for a short period of time, and therefore the arc current flowing through the mechanical switch is commutated to the semiconductor switch for a short period of time, and then said semiconductor switch is actuated again in a current-blocking manner, as a result of which the current conducted in the semiconductor is rapidly reduced to zero without any arcs. Using a hybrid assembly of this type, the effective arc time and therefore the load on the switch can be significantly reduced.
Most hybrid switches require an external power source in order to supply power and to actuate the semiconductor electronic system. This drawback is avoided by the invention described in DE 20 2009 004 198 U1. This is carried out such that the power required to operate the electronic system is drawn from the arc that develops when the mechanical switch is opened. At the same time, a power storage unit, preferably in the form of a capacitor, is charged by the arc current, and then provides the control voltage for shutting off the power semiconductor without any arcs. The switching process in a hybrid switch of this type therefore always involves a switching arc being temporarily formed between the mechanical contacts. However, the drawback of this arrangement is, on one hand, a load on the switchgear as before owing to the contacts burning away (even though this is accordingly reduced due to the significantly shortened arc time) and, on the other hand, a relatively long current load (in particular for higher currents) on the power semiconductor until reliable voltage strengthening has been achieved.
In the circuit described in US 2005/0195550 A1, the power stored in the coil of the electromagnetic drive is used to actuate the semiconductor switch. In order to open the contacts of the switchgear, the power supply to the coil of the drive is shut off. In so doing, the coil releases the energy stored thereby via a Zener diode, which is coupled to the primary side of a transformer. The electrical power flowing through the primary side generates a corresponding voltage on the secondary side of the transformer, which voltage drives a current through a resistor for limiting current and via a Zener diode connected in parallel with the secondary side, which current switches on the semiconductor switch, which is connected in parallel with the contacts of the switchgear and takes over the load current. In so doing, the semiconductor switch is switched on more rapidly than the mechanical contacts of the switchgear are opened, and therefore the load current can already commutate at the moment at which the mechanical contacts on the semiconductor switch open. As a result, a switching arc can in principle be prevented from developing between the mechanical contacts.
The problem addressed by the invention is to propose a switching device for conducting and interrupting electrical currents and a switchgear comprising a switching device of this type, which makes further improvements on the circuit known from US 2005/0195550 A1.